1. Field of the Invention
The present invention relates to a control system for controlling a hybrid vehicle having an internal combustion engine and an electric motor as separate propulsion sources.
2. Description of the Related Art
There have heretofore been known hybrid vehicles each having an internal combustion engine and an electric motor as separate propulsion sources. Japanese laid-open patent publication No. 5-229351, for example, discloses a drive power control system for propulsion sources on such a hybrid vehicle.
The disclosed drive power control system determines an optimum torque for maximizing the efficiency of the engine depending on running conditions of the vehicle, detects an actual drive torque of the engine, and determines an assistive drive torque based on the optimum torque and the actual torque. The drive power control system energizes the electric motor to apply an assistive power at suitable times as when the vehicle is to be accelerated, depending on the assistive drive torque.
The electric motor applies the assistive power by simply adding the output power of the electric motor depending on the remaining capacity of an electric energy storage unit based on the difference between the optimum torque and the actual torque which are generated by a throttle valve opening that is uniquely determined depending on the driver's action to operate the accelerator pedal. For this reason, the drive power control system suffers the following problems:
When the remaining capacity of the electric energy storage unit falls and hence the output power of the electric motor drops, the total drive power, i.e., the sum of the output power from the engine and the output power from the electric motor, is reduced, resulting in a reduction in the actual torque. Accordingly, the drivability of the hybrid vehicle is impaired.
One solution is to replace an ordinary throttle valve mechanically linked to the accelerator pedal with a throttle valve that is electrically controlled by an actuator based on a signal indicative of the amount of operation of the accelerator pedal. When the output power from the electric motor drops, the opening of the throttle valve is controlled to enable the engine to generate an output power commensurate with the drop in the output power from the electric motor. The proposal is effective to prevent the drivability of the hybrid vehicle from being impaired, but is disadvantageous for the following reasons:
One advantage offered by the assistive drive power provided by the electric motor is to reduce the amount of fuel consumption by the engine. Specifically, a demand drive power imposed on a motor vehicle which is propelled by an engine alone is entirely generated by the engine. However, a demand drive power imposed on a hybrid vehicle which is propelled by an engine and an electric motor is partly generated by the engine and partly generated by the electric motor. The greater the amount of the demand drive power shared by the electric motor, the smaller the amount of the demand drive power shared by the engine, i.e., the smaller the amount of work performed by the engine, resulting in a reduction in the amount of fuel consumption by the engine. In the conventional drive power control system described above, the torque generated by the engine is maintained at an optimum level regardless of the remaining capacity of the electric energy storage unit, and hence the advantage of the hybrid vehicle that the amount of work performed by the engine can be reduced to reduce the amount of fuel consumption by the engine is limited.
In the above conventional drive power control system, since the output power of the engine is determined depending on the throttle valve opening, the engine maintains a certain amount of output power at all times except when the throttle valve is fully closed. This mode of operation remains unchanged even when the electric energy storage unit stores an amount of electric energy sufficient enough to enable the electric motor to generate all the demand drive power. Therefore, even when the electric energy storage unit stores such a sufficient amount of electric energy, the electric motor does not generate all the demand drive power, and hence the amount of work performed by the engine cannot be eliminated completely.